LGMS Article of the Month - January

Birthstone of the Month

Garnet, a somewhat uncomplicated aluminum silicate, is an excellent birthstone for January as the cold winter sunsets can be fiery red at times. There are two major groups of garnet: pyralspite (pyrope, almandine, spessartine) and the urgandite (grossular, andradite, uvarovite), but there are also several subspecies. It comes as small as a grain of sand to a stone that is as big as a baseball. Garnet has a hardness of 6.5 to 7.5 on the Mohs Hardness Scale and is 3.4 to 4.3 times as heavy as water. All gem garnets are transparent. They are a combination of several or one of the following: magnesium, iron, manganese, calcium, and vanadium. Pyrope, often found along with diamond, is the only garnet that is always a shade of red and one of the most popular set in jewelry because it is usually clearer and less flawed than almandine. The best stones are a deep blood-like color with no undertones of brown whatsoever. Faceted stones of heavy carat weight are seldom seen at retail; but if so, they are quite costly. Almandine derives its beautiful deep red color mostly from its iron content, but also from other trace elements such as magnesium and chromium. The best specimens show a clear and uniform color, but usually a darker red than pyrope. It is the most commonly seen member of the red garnets and will appear black when viewed from a distance. It also forms as a star garnet. Rhodolite is also a variety of red garnet. It is a mixture of about 55% pyrope and 37% almandine along with other elements. Its color is described as being raspberry, or it can be a darker shade of purple, but raspberry is generally the choice with collectors. Specimens of five or more carats in weight are considered rare. Malaia, a red garnet from Tanzania and Kenya, contains needle-like rutile inclusions and is actually a mixture of pyrope and spessartine, but does not fit in any other red garnet categories.

Spessartine, also known as "Mandarin Garnet" and normally an orange to reddish-orange garnet colored by iron, but at times becomes a darker red or even appears almost black. Mandarin, a vivid pure orange colored gem, is one of the most rare and most sought after of the garnets. Specimens of 10 carats are very unusual. Grossular garnets include tsavorite, hessonite, grossularite, rosalite, blue grossular, and colorless. Tsavorite, the most sought after vivid green variety can rival the natural emerald. Its color is due to vanadium and sometimes chromium. When cut, tsavorite is usually less than three carats. Hessonite, also known as "cinnamon stone," is a brownish-orange color and has been used in jewelry for thousands of years. The clearest stones are the most prized. Grossularite, a gem combining a little more yellow than green, is sometimes mistaken for peridot. Rosalite, a little known pink color and the colors blue and colorless are all very rare and difficult specimens to find. Andradite garnets include demantoid, topazolite, and melanite, which are yellowish-green, brown, and wine-red colored stones. Topazolite is the yellow version, while melanite, containing titanium, is the black variety. Demantoid, the green variety, is the most rare and most valuable of the andradite garnets. Specimens are small, usually about one carat. Its fire is greater than diamond and its best quality green rivals that of emerald. Uvarovite, a constantly beautiful emerald green color, is rarely used as a gem because its crystals are normally too small to be cut.

Localities of garnet are too numerous to mention worldwide.


Sources: Guide to Gems by Oldershaw; Gems by Dennis; Rocks, Gems, & Minerals by Zimm & Shaffer; Gemstones of the World by Schumann

Printed in January 2011 issue of Lowcountry Diggings

 

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